Integrated circuit transporter and a method of communication therefor

ABSTRACT

An improved integrated circuit transponder device is presented comprising, in combination, a single, externally wireless package enclosing an integrated circuit semiconductor chip, a coil within the package being selectively exposed to an electromagnetic field, and a detection portion on the integrated circuit semiconductor chip and being coupled to each end of the coil for detecting when voltages at each end of the coil are approximately equal over a period of time. The detection portion includes an exclusive NOR gate coupled to each end of the coil for determining when the coil voltages are equal, and the output of the exclusive NOR gate is input to a filter before being delivered to the remainder of the device circuitry.

RELATED APPLICATIONS

This patent application is related to pending U.S. patent applicationentitled "A Single-Sided Package Including an Integrated CircuitSemiconductor Chip and Inductive Coil and Method Therefor," filed in thenames of Joseph Fernandez and Lee Furey, and is incorporated herein byreference. This patent application is also related to pending U.S patentapplication entitled "Combination Inductive Coil and Integrated CircuitSemiconductor Chip in a Single Lead Frame Package and Method Therefor,"filed in the names of Lee Furey and Joseph Fernandez, and isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is in the field of wireless radio frequency devices andmethods of communication therefor and, more particularly, is an improvedintegrated circuit transponder device and a method of communicationtherefor.

2. Description of the Related Art

Integrated Circuits (hereafter "ICs") are, of course, well known tothose skilled in the art of electrical engineering. Typically, one willhave a plurality of ICs and other electronic components interconnectedto form an electrical system for performing one or more functions. Inmost electrical systems, ICs are physically connected by way of a numberof external conductors such as wires, thereby permitting each IC tocommunicate with different ICs or other electronic components in thesystem. Alternatively, those skilled in the art are familiar withmethods of communicating with ICs without the use of interconnecting,external conductors such as wires.

In particular, one could form electronic packages enclosing both an ICdie and an inductive coil with the IC's encapsulating material. Thesepackages would have no conductors running externally from the packagefor the purpose of communicating with ICs or other electronic componentsoutside of these packages. Such an externally wireless packageencapsulates both the IC die having the chip logic, and the inductivecoil, which is internally coupled to the IC die. The internal inductivecoil of an externally wireless package generally serves two primaryfunctions. First, when an electromagnetic field is applied through thepackage from an external source such as an external electromagnetictransmission source, a potential is created across the internalinductive coil, and this potential provides current to power theinternals of the package. Second, the inductive coil serves essentiallyas an antenna for transmitting from and/or receiving information for thepackage internals. Such externally wireless packages may be referred toas transponders, meaning that they can both transmit and receiveinformation.

Oftentimes, transponders are referred to as "tags." The term of art,tag, generally refers to externally wireless electronic packages thatare affixed or "tagged" onto merchandise, luggage, or any one of anumber of other objects where item identification is required. It shouldbe pointed out that tags are not only used for item identification; theymay also be used to rapidly report the state of some parameter regardinga particular item. For example, a tag having a temperature or pressuresensor could report the temperature or pressure of the item associatedwith the particular tag. Once a tag is affixed to a given item, it isread with a device typically referred to as a "reader," which sends outa certain excitation frequency that is detected by the tag, and then,when required, responded to by the tag. The reader can read data from atag by detecting perturbations in the electric field caused by a tagtransmission.

One variety of tags would be very simple. Such a simple tag would beidle when there is no reader field present, and when a reader field isasserted, then the tag transmits its data (e.g., item serial number,model number, etc.) continuously until the reader field is turned off.Alternatively, one might require a tag of greater functional capability.Creating a tag of greater functionality, yet limiting its productioncost, generally implies the creation of a tag having minimum on-chiplogic, and a reader with greater functional capability. In this manner,the reader can prompt different actions from the tag simply by sendingdifferent commands to the tag. This approach effectively yields a tagcapable of executing more functions, prompted by different readercommands, with minimum cost of on-chip tag logic. Since the interfacebetween a reader and a tag is usually air, it is very limiting. In otherwords, wireless communication between two elements such as a reader anda tag is relatively more difficult than communication between two otherelements over an electrical conductor such as a wire. In light of themore challenging aspects of wireless communication, the communicationslink between a reader and a tag needs to be sophisticated enough to beable to pass commands back and forth between the two.

One possible manner of communicating with a tag would be to have areader send actual commands to the tag. Such an approach would requirecomplex tag circuitry to enable it to discern between an "empty"electromagnetic field, which is one devoid of any command for the tagother than a simple query for identification data from the tag, and a"loaded" electromagnetic field embedded with one or more commands for atag. Moreover, different electromagnetic field strengths transmittedfrom a reader would necessitate the use of many high gain amplifiers ona tag to enable it to accurately detect commands on top of a carriersignal likely to be changing in orders of magnitude. Readers typicallyhave high gain amplifiers, but placing such high gain amplifiers on atag would require the tag to supply them with levels of power that a tagsimply cannot deliver. Thus, if sending a variety of different commandsto a tag is prohibitive because of tag power supply limitations, then apreferred communication technique would simply be to interrupt theelectromagnetic field transmitted from the reader for a relatively shortperiod of time in order to signal to the tag that it is then to commencea particular operation.

This interruption in the transmission of electromagnetic radiation froma reader to a tag is referred to as a "gap." In the simplest mode ofoperation, it is desirable that a tag be able to detect a single gap inthe transmission from the reader in order to trigger a particularoperation by the tag. However, in order to have a tag with greateroperational capability, it must be able to detect multiple gaps in theelectromagnetic field transmitted by the reader. More specifically, morecapable tags will be able to recognize certain combinations of gapsestablished over certain time intervals, thereby triggering differentoperations from the tag. It is important to recognize that, in general,tags are powered only by the electromagnetic field inducing potentialacross the tag's internal coil, thereby producing current for the tag'sinternal logic. Now, it is also important to recognize that the internaltag circuitry has associated with it some capacitance capable ofretaining some power for the tag logic. However, the power retainingcapability of most tags is very limited, and therefor, it is necessaryto refresh the tag's capacitance-retained power by recurring applicationof the reader's electromagnetic field. In order for a tag to be able todetect combinations of gaps, the reader's electromagnetic field must beapplied, interrupted, and reapplied a number of times over particulartime intervals. However, because the tag's capacitance-retained powersupply must be regularly refreshed from the reader's electromagneticfield, the duration of any gap must be relatively short.

The previous discussion under the heading, "Description of the RelatedArt" is largely background information regarding certain aspects of theoperation of a reader and a tag. Regarding the state of prior art tags,they detected gaps by determining when the potential at both ends of thetag's internal coil would be at or near chip ground. The term chipground refers to a ground signal produced by the electronics on the ICdie internal to the tag. More specifically, certain components of thetag's IC die process the signals coming from each end of the tag's coilin order to produce a ground signal for use by the rest of the internaltag circuitry. When prior art tags detected the condition where thepotential at both ends of the tag's coil attained, or at leastapproached, chip ground at the same time, the tag circuitry detected agap.

One problem with detecting this condition was that the coil voltageswere measured relative to chip ground. The tag's internal circuitry mustrun off of some ground (i.e., chip ground) derived from the tag's coil,and that ground level depends on many factors. For example, when the tagis in a reader's electromagnetic field, the chip ground level issomewhat predictable because there is a rectifier in the tag's internalcircuitry which references chip ground to the lower potential of the twocoil terminals. Herein lies the problem. When the amplitude of the coilvoltages goes to zero, the aforementioned rectifier essentially becomesan open circuit, and the coil is floating with respect to the chipground. Of course, the capacitance of the tag's internal circuitry helpsto maintain the chip ground signal; however, the reality of thesituation is that when the reader's electromagnetic field isinterrupted, and the coil voltages go to zero, the rectifier becomes anopen circuit. Consequentially, the chip ground signal is toounpredictable to be effectively used as a reference to measure thepotential of the coil terminals. As a result, the detection of a gap ishampered under the prior art approach.

Therefore, there existed a need to provide an improved tag capable ofdetecting a gap in a reader's electromagnetic field independent of thechip ground signal.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved integratedcircuit transponder device and an improved method of communicationtherefor.

Another object of the present invention is to provide an improvedintegrated circuit transponder device capable of detecting gaps in thetransmission of an electromagnetic field in proximity thereto, therebytriggering the device into one or more operations.

Yet another object of the present invention is to provide an improvedintegrated circuit transponder device capable of detecting,independently of a ground signal for the device, when induced voltageson the ends of an internal coil are approximately equal over a period oftime.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to one embodiment of the present invention, an integratedcircuit transponder device is disclosed comprising, in combination, asingle, externally wireless package enclosing an integrated circuitsemiconductor chip, a coil within the package being selectively exposedto an electromagnetic field, and detection means on the integratedcircuit semiconductor chip and being coupled to each end of the coil fordetecting when voltages at each end of the coil are approximately equalover a period of time. The detection means, independent of a groundsignal on the integrated circuit semiconductor chip, detects aninterruption of transmission of the electromagnetic field. The devicefurther includes a capacitor coupled in parallel to the coil.Additionally, the integrated circuit semiconductor chip includes arectifier coupled to each end of the coil for rectifying signals fromthe coil, and the rectifier includes a pair of diodes and a pair oftransistors. The detection means includes an exclusive NOR gate coupledto each end of the coil. Moreover, one end of the coil is input to afirst pair of serially connected inverters having an output connected toa first input of the exclusive NOR gate, and another end of the coil isinput to a second pair of serially connected inverters having an outputconnected to a second input of the exclusive NOR gate. The output of theexclusive NOR gate is input to a filter before being supplied to therest of the device's circuitry.

According to another embodiment of the present invention, a method ofoperating an integrated circuit transponder device is disclosedcomprising the steps of providing a single, externally wireless packageenclosing an integrated circuit semiconductor chip, providing a coilwithin the package being selectively exposed to an electromagneticfield, and providing detection means on the integrated circuitsemiconductor chip and being coupled to each end of the coil fordetecting when voltages at each end of the coil are approximately equalover a period of time. The detection means, independent of a groundsignal on the integrated circuit semiconductor chip, detects aninterruption of transmission of the electromagnetic field. This methodfurther includes the step of providing a capacitor coupled in parallelto the coil. The integrated circuit semiconductor chip includes arectifier coupled to each end of the coil for rectifying signals fromthe coil, and the rectifier includes a pair of diodes and a pair oftransistors. The step of providing the detection means includes the stepof providing an exclusive NOR gate coupled to each end of the coil.Moreover, one end of the coil is input to a first pair of seriallyconnected inverters having an output connected to a first input of theexclusive NOR gate, and another end of the coil is input to a secondpair of serially connected inverters having an output connected to asecond input of the exclusive NOR gate. Prior to being supplied to theremainder of the device's circuity, an output of the exclusive NOR gateis input to a filter.

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following, more particular,description of the preferred embodiments of the invention, asillustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a timing diagram showing the induced voltages on the tag'scoil.

FIG. 2 is a timing diagram showing the induced voltages on the tag'scoil, and the output of the tag's rectifier.

FIG. 3 is a timing diagram showing the effect on the induced voltages inthe tag's coil and on the lower voltage supplied from the rectifiercaused by the assertion of a gap in the electromagnetic field from areader.

FIG. 4 is a simplified electrical schematic of the improved tag.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 4, an integrated circuit transponder device (hereaftermore simply referred to as "tag") is shown and generally designated byreference number 10. The tag 10 is enclosed in a single package in amanner well known to those skilled in the art. The package is notexplicitly shown in the Figure for clarity of presentation; however, thepackage would enclose all elements of the tag 10, and the package wouldbe externally wireless. This means that the package would envelop everyelement in FIG. 4 except for element number 12, which will be discussedbelow. Note also that FIG. 4 shows two vertical lines labeled 22, whichare not part of the tag 10. Rather, the lines 22 are shown toconceptually demarcate the Integrated Circuit (hereafter "IC")semiconductor chip of the tag 10 from the tag components distinct fromthe IC semiconductor chip. Those portions of the Figure shown to theright of vertical lines 22 comprise the die elements of the ICsemiconductor chip, and the portions to the left of vertical lines 22(except for element 12 which is not part of the tag 10) comprise tagelements not located on the tag's IC semiconductor chip. Summarizing forthe sake of absolute clarity, all elements in FIG. 4 except thoselabelled 12 and 22 comprise the tag 10, and the tag elements located tothe left of vertical lines 22 are not located on the tag's ICsemiconductor chip, while those to the right of vertical lines 22 arepart of the tag's IC semiconductor chip. Lastly, it should be pointedout that the particular details regarding the fabrication of a tag, suchas tag 10 shown in FIG. 4, are more explicitly delineated in the relatedpatent applications mentioned above.

Still with reference to FIG. 4, the tag 10 comprises, in combination, asingle package (not explicitly shown) enclosing an IC semiconductor chip(i.e., that portion of tag 10 to the right of vertical lines 22), a coil14 within the package being selectively exposed to an electromagneticfield 12, and a detection portion (to be discussed in detail below) onthe IC semiconductor chip and being coupled to each end of the coil 14for detecting when voltages at each end of the coil 14 are approximatelyequal over a period of time. The electromagnetic field 12 is generatedby a device well known to those skilled in the art as a reader (notshown). The reader transmits the electromagnetic field 12 to penetratethe coil 14, and selective interruptions or gaps in the transmission ofthe electromagnetic field 12 are intended to be detected by the tag 10.A capacitor 16 is coupled in parallel to the coil 14. Penetration of theelectromagnetic field 12 through the coil 14 will induce coil voltagehaving a resonant frequency that is the result of the values of coil 14inductance and capacitor 16 capacitance. Thus, the capacitor 16 ismatched with the coil 14 in a manner well known to those skilled in theart to develop the desired resonant frequency for the tag 10.

Two induced voltages A and B taken off the ends of the coil 14 aresupplied on chip (i.e., to the right of vertical lines 22) to arectifier 24. The rectifier 24 includes a pair of diodes 26 and 28 and apair of PMOS transistors 30 and 32 coupled to perform the desiredrectification of signals A and B. Note that other types of rectifierswell known to those skilled in the art could be implemented, if desired.The rectifier 24 here has the anode junctions of the diodes 26 and 28tied together at a node providing a signal DC-, and the drain junctionsof PMOS transistors 30 and 32 are tied together at a node providing asignal DC+. Note that the signals DC- and DC+ are provided to the block48, labeled "Rest of Circuit." Block 48 represents circuitry performingtag functions, of which there are many possibilities now known or to belearned by those skilled in the art in the future. DC- and DC- representa power and ground supply for block 48, respectively. Returning to therectifier 24, the source junction of PMOS transistor 30 and the cathodejunction of diode 28 are tied together at a node supplied with thesignal A from one end of the coil 14. Similarly, the source junction ofPMOS transistor 32 and the cathode junction of diode 26 are tiedtogether at a node supplied with the signal B from another end of thecoil 14. Additionally, the gate junctions of PMOS transistors 30 and 32are tied to nodes having signals B and A, respectively. With thatstructure for rectifier 24, it provides power supply and ground signals,DC+ and DC-, respectively, to block 48. It should also be pointed outthat the ends of the coil 14 are coupled to the block 48 to provideinduced signals A and B to the block 48, which includes circuitry wellknown to those skilled in the art to produce a clock signal for block 48from the processing of signals A and B.

Still referring to FIG. 4, the ends of the coil 14 are coupled to thedetection portion to provide signals A and B thereto. The detectionportion includes an exclusive NOR gate 44 coupled to each end of thecoil 14 through two pairs of serially connected inverters 36-42. Inparticular, one end of the coil 14 is coupled to inverter 36 to providesignal A to its input, and its output feeds the input of seriallyconnected inverter 38, which has its output connected to an inputjunction for exclusive NOR gate 44. Similarly, another end of the coil14 is coupled to inverter 40 to provide signal B to its input, and itsoutput feeds the input of serially connected inverter 42, which has itsoutput connected to another input junction for exclusive NOR gate 44.The inverters 36 and 40 are level shifting inverters well known to thoseskilled in the art. They simply downshift the input voltages of signalsA and B down to a lower level for processing by the exclusive NOR gate44, a filter 46, and the remainder of the tag's circuitry in block 48.The inverters 38 and 42 are non-level shifting inverters (i.e., genericinverters). They simply ensure that the output of each seriallyconnected inverter chain has the same polarity as its respective input;however, as the detection portion implements an exclusive NOR gate 44,it will become apparent later in the section discussing the operation ofthe tag 10 that the inverters 38 and 42 could be removed if desiredwhile maintaining the same operation of the tag 10. The output of theexclusive NOR gate 44 is input to a filter 46 such as an RC-type filterwell known to those skilled in the art, and the output of the filter 46is delivered to the remaining tag circuitry in block 48.

OPERATION

Recalling from the earlier discussion with respect to FIG. 4, acapacitor 16 is selected with a value of capacitance that taken with theinductance associated with the coil 14 results in obtaining the desiredresonance frequency for the tag 10. In the current embodiment of the tag10, that resonant frequency is generally chosen to fall within the rangeof 125 KHz to 13.5 MHz; however, those skilled in the art recognize thatvirtually any resonant frequency desired can be attained by anappropriate combination of the coil 14 and capacitor 16. Regarding block48 from FIG. 4, it should be pointed out that there are a variety oftags capable of performing a variety of different functions. A fewexamples of the potential uses for tags include pet identification tags,personal identification tags, and luggage identification tags. Thus, onecan see that one function of a tag is to provide a method of identifyingthe object or person associated with a particular tag. Anotherfunctional aspect of tags is that they are contactless, meaning that noexternal conductors are required for the tag to be in communication withan external system component such as a reader. That is, a readerprovides power to a tag, and it communicates with a tag, without the useof interconnecting wires. Yet another feature of tags is what thoseskilled in the art refer to as the "anti-collision" function. Thisfunction is useful under the following circumstances. Suppose one has anumber of tags, each being exposed to the electromagnetic field 12 beinggenerated by a reader; however, the reader is only interested incommunicating with a particular "target" tag in the group. The"anti-collision" feature permits non-targeted tags to remain within thereader's field 12 without responding to the reader's prompts, therebypermitting the non-targeted tags to maintain their field-induced power,while the reader communicates with the targeted tag. These and other tagfeatures can be implemented, in any one of a number of manners wellknown to those skilled in the art, with logic in the block labeled 48.The particular circuitry required to perform such features is not shownin block 48 of the tag 10 as they are not the focus of this discussion.Rather, the focus here is to disclose an improved tag 10 and an improvedmethod of communication therefor with a reader, centering on the tag 10detecting a gap in the reader's electromagnetic field 12.

Referring now to FIG. 1, two signals A and B are shown. When the reader(not shown) transmits an electromagnetic field 12 (see also FIG. 4regarding this discussion) that penetrates the coil 14 in tag 10, apotential is induced in the coil 14, and the voltage signals from theends of the coil 14 are represented by signals A and B. The signals aregenerally of opposing polarity, as can be seen by viewing points in timelabelled T₁ and T₂. At time T₁, signal A is at a maximum positivepotential, while signal B is at a maximum negative potential.Conversely, at time T₂, signal B is at a maximum positive potential,while signal A is at a maximum negative potential.

Referring to FIG. 2, the two signals A and B from FIG. 1 are againshown, as well as additional signals DC+ and DC-. The signals A and Bare, as before, the induced voltage signals from the ends of the coil 14(see also FIG. 4 regarding this discussion). Signals DC+ and DC- areproduced from the application of signals A and B to the rectifier 24.The rectifier 24 operates to produce DC+ such that it follows the higherof signal A or signal B. Also, note that the signal DC+ is shownseparated from the higher of signal A or signal B; however, thisseparation is exaggerated for the sake of visual clarity. In practice,DC+ nearly identically follows the higher of signals A or B.

Still regarding the operation of rectifier 24, at time T₁, signal A ishigher than signal B. Also, the gate of PMOS transistor 30 is tied tosignal B, while its source is tied to signal A. Whenever, the gate of aP-channel MOS transistor is lower than its source or drain, it will beon. Accordingly, PMOS transistor 30 will be on at time T₁, while PMOStransistor 32 will be off since its gate, coupled to signal A, is higherthan its source coupled to signal B. Thus, at time T₁, the currentassociated with signal A will conduct through PMOS transistor 30 to theDC+ node, and the DC+ signal will follow signal A. Conversely, at timeT₂, signal B is higher than signal A, and the gate of PMOS transistor 32is tied to signal A, while its source is tied to signal B. Again,whenever the gate of a P-channel MOS transistor is lower than its sourceor drain, it will be on. Accordingly, PMOS transistor 32 will be on attime T₂, while PMOS transistor 30 will be off since its gate, coupled tosignal B, is higher than its source coupled to signal A. Thus, at timeT₂, the current associated with signal B will conduct through PMOStransistor 32 to the DC+ node, and the DC+ signal will follow signal B.In this manner then, the signal DC+ will approximately follow the higherof signals A and B. Ultimately, the signal DC+ is sufficient to providea DC power supply for the circuitry of block 48.

Still referring to FIG. 2, at time T₁, signal A is greater than B. Ifsignal B is also low relative to DC- then diode 26 (again see FIG. 4regarding this discussion) will be forward biased and the signal DC-will be drawn down toward the level of signal B (i.e., it will be drawndown to a diode drop above B); however, signal A won't have any effectsince diode 28 will be reversed biased. DC- will approximately follow adiode drop above the lowest potential between signals A and B. Thusproceeding forward from T₁, signal DC- will follow signal B upward; thisis because of the coupling capacitance of diode 26. Immediatelyfollowing the midway point between times T₁ and T₂, signal A becomeslower than signal B. Soon after that point, the diodes 26 and 28 switchtheir biases between forward and reverse. Specifically, approaching timeT₂, signal B is greater than A. Then, if signal A is low relative toDC-, then diode 28 will be forward biased and the signal DC- will bedrawn down toward the level of signal A (i.e., it will be drawn down toa diode drop above A); however, signal B won't have any effect sincediode 26 will be reversed biased. In this manner, the signal DC- willapproximately follow, at a diode drop above, the lower of signals A andB. The signal DC- is sufficient to provide a ground supply for thecircuitry of block 48.

Before moving onto the operation of the detection portion of the tag 10,it is important to note again that the tag 10 contains no internal powersupply and no external conductors either for delivering power to the tag10 or permitting the tag 10 to communicate with outside devices via suchdirect conductors. Yet the tag 10 has power and it can communicate withoutside devices such as a reader. The tag 10 has power because itproduces its internal operating power, DC+ and DC-, from the voltages, Aand B, induced on its coil 14 when the reader transmits anelectromagnetic field 12. Additionally, the tag 10 has some internalcapacitance permitting it to retain, for a certain period of time, itsinduced power. Moreover, the tag 10 can communicate with externaldevices like the reader without the use of interconnecting wires, andthis is because the tag 10 uses its coil 14 essentially as an antenna toboth receive and transmit electromagnetic signals carrying information.Lastly, it should be mentioned again that the signals A and B aredirectly input to the block 48, which processes these signals in amanner well known to those skilled in the art in order to provide aclock signal for the circuity in block 48.

Referring to FIG. 3, signals A, B, and DC- are shown, and they areproduced as discussed above. In FIG. 3 however, the electromagneticfield 12 coming from the reader has been momentarily interrupted to forma gap in field transmission. The left edge of the region labeled "gapasserted" indicates the time at which the gap in field transmissionbegins, while the right edge of this region delineates that point intime when the field 12 from the reader returns. Note that when the gapcommences that the signals A and B do not instantaneously go to zero.Similarly, when the reader's field transmission recommences, the signalsA and B do not immediately reach their maximum values. The lag times inthe fall and rise of signals A and B are primarily attributable to thecapacitor 16 and other impedance looking into the tag 10. Regardless ofthe delay in the decay of signals A and B, they ultimately steady out atthe same value, which is indicative of a gap in the field 12. However,to properly detect this steady-state condition of signals A and B, onegenerally requires a reference signal, namely ground or DC-. Thispresented a problem.

In particular, during the field gap, signals A and B will eventuallyapproximately equal each other. At or near this point in time, there isno potential difference between signals A and B across the rectifier 24.Then, the rectifier 24 is essentially an open circuit because whensignals A and B are equal, or approximately so, neither diode 26 nordiode 28 will be forward biased, and there will be no PMOS transistor(30 or 32) gates lower than their respective sources or drains, so nocurrent will flow through rectifier 24. At this point, DC- or ground isno longer supplied to block 48. Of course, as mentioned before, block 48has some internal capacitance permitting it to retain some amount ofpower, DC+, and ground DC-. In practice however, predicting the level ofground or DC- when the rectifier 24 is an open circuit is verydifficult, and accordingly determining when signals A and B are both atground (i.e., detecting a gap) is enigmatic since the level of ground isunpredictable at this time. Therefor, there was a need to develop a tag10 capable of detecting a gap in the reader's electromagnetic field 12,independent of a ground signal on the integrated circuit semiconductorchip of the tag 10. The solution lies in the incorporation of thedetection portion to the tag 10.

Referring to FIG. 4, the signals A and B are input to level shiftinginverters 36 and 40 that shift the voltage level of their respectiveinputs to a high or low level usable by the downstream circuitry. Morespecifically, if signals A and B are above a certain magnitude, then theoutput of level shifting inverters 36 and 40 will be at the high levelusable by downstream circuitry, but if signals A and B are below acertain magnitude, then the output of level shifting inverters 36 and 40will be at the low level usable by the downstream circuitry. Animportant point to note is that when there is a gap in the reader'selectromagnetic field 12, signals A and B will ultimately reachapproximately the same level for some period of time (see FIG. 3). Atthat point in time, the outputs of level shifting inverters 36 and 40will be the same--either both high or low. The outputs of level shiftinginverters 36 and 40 are fed through inverters 38 and 42, respectively,to arrive at the same polarity of signals leaving inverters 38 and 42 aswas entering corresponding level shifting inverters 36 and 40.

The outputs of inverters 38 and 42 are input to exclusive NOR gate 44which will output a high signal when its input signals are the same, anda low signal when different. This means that the exclusive NOR gate 44outputs a high signal when the signals A and B are the same, or nearlyso, during a gap in the field 12. Simply put, the exclusive NOR gate 44outputs a high signal when a gap is detected. Those skilled in the artrecognize that because an exclusive NOR gate 44 is used, inverters 38and 42 may be omitted from the tag 10, if desired since the exclusiveNOR gate 44 would still output a high signal in the presence of a gap.The filter 46 comprises an RC type filter. It is necessary becausesignals A and B do equal each other for a very short period of time whenthey cross (see FIGS. 1-3). The filter 46 prevents any high signal fromthe output of the exclusive NOR gate 44 passing to block 48 at thesetimes; however, during a gap in the field 12, signals A and B equal eachother for a longer period of time. In this later case, a high outputfrom the exclusive NOR gate 44 will not be filtered by filter 46, and ahigh signal will continue on to the circuitry in block 48, therebyindicating the presence of a gap in the transmission of theelectromagnetic field 12 from the reader. In response, circuitry wellknown to those skilled in the art within block 48 performs the promptedtag operation. Note also that the tag 10 could detect series orcombinations of gaps in the field 12 in order to trigger differentresponses from the tag 10. Transmissions from the tag 10 back to thereader are initiated by circuitry within block 48. There are a number ofdifferent ways to initiate such transmissions, all well known to thoseskilled in the art. Lastly, the tag's transmission data coming fromblock 48 is routed through other lines (not shown) to the ends of thecoil 14, acting at this stage as a transmitting antenna, to send anelectromagnetic transmission to the reader.

Although the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that changes in form and detail may be madetherein without departing from the spirit and scope of the invention.For example, an exclusive NOR gate 44 is used here to detect a gap inthe reader's electromagnetic field 12; however, if desired, one couldmodify the tag 10 to implement an exclusive OR gate, or the like.

What is claimed is:
 1. An integrated circuit transponder devicecomprising, in combination:a single package enclosing an integratedcircuit semiconductor chip; a coil within said package being selectivelyexposed to an electromagetic filed; and detection means on saidintegrated circuit semiconductor chip and being coupled to each end ofsaid coil for detecting when voltages at each end of said coil areapproximately equal over a period of time, said detection means includesan exclusive NOR gate coupled to each end of said coil; wherein one endof said coil is input to a first pair of serially connected invertershaving an output connected to a first input of said exclusive NOR gate,and wherein another end of said coil is input to a second pair ofserially connected inverters having an output connected to a secondinput of said exclusive NOR gate.
 2. A method of operating an integratedcircuit transponder device comprising the steps of:providing a singlepackage enclosing an integrated circuit semiconductor chip; providing acoil within said package being selectively exposed to an electromageticfield; and providing detection means on said integrated circuitsemiconductor chip and being coupled to each end of said coil fordetecting when voltages at each end of said coil are approximately equalover a period of time, said step of providing said detection meansincludes the step of providing an exclusive NOR gate coupled to each endof said coil; wherein one end of said coil is input to a first pair ofserially connected inverters having an output connected to a first inputof said exclusive NOR gate, and wherein another end of said coil isinput to a second pair of serially connected inverters having an outputconnected to a second pair of said exclusive NOR gate.